The present invention relates generally to electric machines and, in particular, to a stator winding for an electric machine having a plurality of filars with reduced cross current circulation. Electric machines, such as alternating current electric generators, or alternators are well known. Prior art alternators typically include a stator assembly and a rotor assembly disposed in an alternator housing. The stator assembly is mounted to the housing and includes a generally cylindrically-shaped stator core having a plurality of slots formed therein. The rotor assembly includes a rotor attached to a generally cylindrical shaft that is rotatably mounted in the housing and is coaxial with the stator assembly. The stator assembly includes a plurality of wires wound thereon, forming windings. The stator windings are formed of straight portions that are located in the slots and end loop sections that connect two adjacent straight portions of each phase and are formed in a predetermined multi-phase (e.g. three or six) winding pattern in the slots of the stator core. The rotor assembly can be any type of rotor assembly, such as a “claw-pole” rotor assembly, which typically includes opposed poles as part of claw fingers that are positioned around an electrically charged rotor coil. The rotor coil produces a magnetic field in the claw fingers. As a prime mover, such as a steam turbine, a gas turbine, or a drive belt from an automotive internal combustion engine, rotates the rotor assembly, the magnetic field of the rotor assembly passes through the stator windings, inducing an alternating electrical current in the stator windings in a well known manner. The alternating electrical current is then routed from the alternator to a distribution system for consumption by electrical devices or, in the case of an automotive alternator, to a rectifier and then to a charging system for an automobile battery.
One type of device is a high slot fill stator, which is characterized by rectangular shaped conductors that are aligned in at least one radial row in each slot and whose width, including any insulation, fits closely to the width of the rectangular shaped core slots, including any insulation. It is obvious to those skilled in the art that the term rectangular shaped core slot may include a rectangular shape with radii at the corners and/or include a specially shaped slot opening at the inner surface.
High slot fill stators are advantageous because they have less heat dissipation with lower electrical resistance and help produce more electrical power per winding than other types of prior art stators. These stators, however, are disadvantageous because the windings are typically interlaced, in which the wires are required to alternate outer and inner radial portions of each slot. This is because one end loop connects the straight segment housed in an outer radial depth of the first slot to a straight segment housed in an inner radial depth of the second slot. This conductor leaves a void in the outer radial depth of the second slot, therefore a second conductor must connect the straight segment housed in an outer radial depth of the second slot to a straight segment housed in an inner radial depth of the third slot. These interlaced windings require an interlacing process to interlace the conductors of all the phases prior to inserting the winding into the core or a connection process to connect the individual U-shaped conductors and therefore disadvantageously increase the complexity of placing the winding the stator.
In a bi-filar winding stator each phase turn includes two wires or filars, which are connected in parallel. The wire cross section in a bi-filar design, having a certain electrical resistance, is substantially half of that in a single-filar design, having substantially the same electrical resistance. Therefore, wires in bi-filar designs are much more structurally flexible for bending and turning at stator winding end turns. However, bi-filar windings may be subject to overheating due to cross current circulation between filars. Cross current circulation can occur when the two filars are linked by a different amount of flux and therefore have different generated voltages. This can occur if the straight segments of the first filar have a different average radial position in the core slots than the straight segments of the second filar and a phenomenon known as magnetic flux slot leakage, is present.
The “normal” path of the magnetic flux is to encircle completely around a core slot by traveling radial outward down one tooth, circumferentially across the yoke and finally radially inward down another tooth. This path for the magnetic flux encircles and therefore links all of the straight segments located in the encircled core slot. However, some amount of the magnetic flux short circuits this path by prematurely crossing the slot before it reaches the yoke—this portion of the magnetic flux is known as slot leakage flux. This slot leakage flux only encircles, and therefore links, straight segments that are located radially inward of the radial position where it pre-maturely crosses the slot. Therefore, slot leakage flux can cause filars with different average radial positions in a slot to have different generated voltages and therefore cross current circulation. This over-heating and cross current circulation reduces the efficiency of the alternator.
In addition, the cross circuit circulation problem is magnified for high slot fill stators whose typical circumferential core slot width, such as less than 2.6 mm, is narrower than the typical stator core slot width. This is true because magnetic flux prefers to follow the path of least resistance, which is normally along the path of magnetic permeable material as previously mentioned as the “normal” path. However, depending on the magnetic reluctance of the path across a core slot, some amount of magnetic flux prematurely crosses the core slot. The width of a non-permeable material, such as the air, copper wire and insulator, found in a core slot increases the magnetic reluctance to allow magnetic flux to flow. Therefore, the circumferentially narrower the core slot, the larger the amount of flux leakage and the larger the amount of cross circuit circulation.
Recent stator innovations have increased the number of phases in a stator from three phases to a larger number of phases, such as six. A stator having a higher number of phases consequently magnifies the cross circuit circulation problem because the stator has a higher number of core slots located in a core having a similar circumference distance and therefore the circumferential width of each core slot is typically narrower.
It is desirable, therefore, to provide a stator that utilizes a bi-filar design while reducing the amount of cross current circulation between filars.